1. Field of the Invention
The invention relates to a printed wiring board, and more particularly to a printed wiring board including a circuit wire electrically connected at a distal end thereof to a pad formed on the printed wiring board (hereinafter, simply referred to as “pad”).
2. Description of the Related Art
FIG. 7 is a plan view illustrating an example of a conventional printed wiring board.
A plurality of circuit wires 1 is arranged on a substrate in parallel with one another. Each of the circuit wires 1 extends in a certain direction X. Each of the circuit wires 1 has a circuit pad 34 at a distal end. An electrical insulator 3 such as a solder resist or pre-preg covers the substrate and the circuit wires 1 therewith.
The electrical insulator 3 is formed with a plurality of circular openings 32 through each of which the pad 34 is entirely exposed.
Since the pad 34 is entirely exposed through the circular opening 32 in the conventional printed wiring board illustrated in FIG. 7, the electrical insulator 34 does not exist above the pad 34, and hence, the pad 34 is not compressed by the electrical insulator 3. As a result, the pad 34 makes contact with the substrate with insufficient contact force, and hence, the conventional printed wiring board illustrated in FIG. 7 is accompanied with a problem that the pad 34 is likely to peel off at the circular opening 32.
As one of solutions to the problem, the circular opening 32 may be formed smaller than the pad 34. By forming the circular opening 32 smaller than the pad 34, the pad 34 is compressed at its periphery by the electrical insulator 3, resulting in that the pad 34 makes contact with the substrate with sufficient contact force, and hence, it is possible to prevent the pad 34 from peeling off at the circular opening 32.
However, a contact composed of solder or paste containing electrical conductor, formed on the pad 34, makes contact only with a surface of the pad 34, and does not make contact with a sidewall of the pad 34; resulting in another problem of insufficient contact force between the pad 34 and a contact formed on the pad.
Furthermore, since the circular pad 34 is formed at a distal end of the circuit wire 1 in the conventional printed wiring board illustrated in FIG. 7, a pitch between adjacent pads 34 is unavoidably small. An area array type LSI package most often used presently is designed to have a lot of terminals at a small pitch in accordance with a need that electronic devices are mounted at a high density. Thus, when wires are connected to the terminals of such an area-array type LSI package, it would be quite difficult to increase the number of wires extending through a gap between the adjacent pads.
Japanese Utility Model Application No. 63-126428 has suggested a printed wiring board including an electrically insulating substrate, a plurality of pads formed on the substrate, and solder resist thicker than an electrical conductor covering the pads at ends therewith, wherein the pads have a width greater than a width of the solder resist.
Japanese Patent Application Publication No. 8-64939 has suggested a printed wiring board on which a device including a plurality of elongate terminals is to be mounted, including a substrate, a plurality of solder pads arranged on an upper surface the substrate for soldering the terminals therewith, and solder resist formed on the substrate in an area except the solder pads for preventing adhesion of molten solder. The substrate is formed on a lower surface with a region in which solder resist is not formed for absorbing extra molten solder.
Japanese Patent Application Publication No. 8-107264 has suggested a printed wiring board including a substrate, a terminal formed on the substrate, and an electrically insulating layer covering the terminal therewith. The electrically insulating layer is formed with an opening through which a principal surface area of the substrate except an edge of the terminal is exposed, and is thicker than the terminal.
Japanese Patent Application Publication No. 9-27661 has suggested a printed wiring board on which a plurality of electrodes is arranged at a predetermined pitch. Each of the electrodes is comprised of a main body having a certain shape, and an extended portion wider than the main body in a first direction in which the electrodes are arranged. The extended portions of the electrodes disposed adjacent to each other are spaced away from each other in a second direction perpendicular to the first direction.
Japanese Patent Application Publication No. 2000-77471 has suggested a substrate on which an electrically conductive pattern on which a bump of an electronic device is mounted in flip-chip is formed. The electrically conductive pattern is comprised of a wire pattern which will make a wire, and a pad formed integrally with the wire pattern at a location at which the bump is mounted. The electrically conductive pattern has a width W2 greater than a width W1 of the pad.